I wonder if he might have some of the characteristics of an only child. Do we have any sense of what the age gap is between him and Elinor? The fact that she has so many first-born characteristics makes me think he wasn’t around much while the girls were being raised. He might have spent some years as an only child and then (as other’s have theorized) he might have been away at school much of the time when his half-sisters were growing up.